A seam-welding method is known for joining workpieces together by forming a plurality of nuggets continuously in an overlapping manner between the workpieces in the form of a stacked assembly in which plural workpieces made up from metal plates or the like are stacked. In such a seam-welding method, current is supplied intermittently to a pair of roller electrodes that sandwich and apply pressure to the stacked assembly while the rollers are made to rotate. More specifically, as in the energizing (i.e., current supplying) cycle shown in FIG. 6A, the roller electrodes are moved relatively with respect to the stacked assembly while repeatedly performing, as one cycle (e.g., 60 msec), an energizing period (e.g., 24 msec) during which current is supplied between the roller electrodes to energize the electrodes, and an interruption period (e.g., 36 msec) during which the supply of current to the roller electrodes is interrupted.
During the energizing period, resistance heating (Joule heat) is generated in the vicinity of contact surfaces of the workpieces that are pressed by the roller electrodes, and a molten portion is formed therein. In addition, during the interruption period, a location of the stacked assembly where the molten portion has been formed is cooled while being pressed by the roller electrodes, whereby the molten portion becomes solidified to form a nugget. As a result, in the foregoing manner, nuggets are formed continuously in a mutually overlapping manner between the workpieces, and seam welding of the stacked workpiece is thereby carried out.
Incidentally, recently, for example, it has been proposed to mount a seam-welding device on which roller electrodes are provided on a robot, and to carry out seam welding of large scale workpieces such as component parts for an automotive body. In this case, it is sought to increase the welding speed without increasing the size and scale of the seam-welding device.
However, if the relative speed of movement of the roller electrodes is increased in order to increase the welding speed, there is a concern that the roller electrodes may become separated or distanced from the molten portion before the molten portion has solidified adequately during the interruption period. In this case, the molten portion becomes solidified under a condition in which the pressing force by the roller electrodes is reduced or is not applied at all, and upon solidification of the molten portion, it is likely for cracks to develop due to volumetric contraction or the like.
As a means for bringing about solidification of the molten portion promptly prior to moving the roller electrodes, it may be considered to cool the molten portion through use of cooling water. However, in this case, there is a need to provide cooling equipment including piping or the like for the cooling water. Further, if a condition is left in which the cooling water remains adhered to the automotive body or the component parts, since there is a concern that rust or the like may occur, in the case that the adhered cooling water is removed, further additional equipment is required. Consequently, from the standpoint of avoiding an increase in size of the seam-welding device, cooling of the molten portion using cooling water is not preferred.
Thus, as shown in FIG. 6B, along with increasing the welding speed, it has been considered to shorten one cycle (e.g., 48 msec), together with increasing the proportion of the interruption period (e.g., 32 msec) within the one cycle. In accordance therewith, the proportion of the interruption period is increased relative to the energizing period (16 msec), and the timing at which solidification of the molten portion begins can be hastened or made to occur sooner. As a result, it is possible for the molten portion to become solidified prior to moving the roller electrodes, or stated otherwise, under a condition in which the pressing force is applied to the molten portion.
However, as described above, when the proportion of the energizing period is decreased, and the proportion of the interruption period is increased within one cycle, there is a concern that the stacked assembly may be subjected to cooling more than necessary. As a result, due to the fact that electrical resistance in the stacked assembly is decreased, the required melting current for the purpose of forming the molten portion is made greater, and it is likely for spatter to occur. Further, the temperature difference of the stacked assembly between the energizing period and the interruption period increases, and due to the fact that a large volumetric change therein occurs, ultimately, it becomes difficult for cracks or the like to be suppressed sufficiently.
In order to suppress cooling of the stacked assembly more than necessary, it may be considered to adopt the seam-welding method disclosed in Japanese Laid-Open Patent Publication No. 11-058026. More specifically, seam welding is carried out in which a first heat-inputting welding current P, which is capable of forming a molten portion, and a second welding current B, the heat-input of which is smaller than that of the first welding current P, are supplied alternately, and an interruption period during which the welding current is interrupted is not provided.